GPS Systems have been adapted in the prior art to track objects and vehicles. All-weather GPS systems, which have been designed and implemented by the United States Department of Defense, are intended to be comprised of 24 satellites (21 of which shall be active and 3 of which will be on standby), ground control stations, and individual GPS receiver units throughout the world. The satellites are placed in elliptical orbits and are evenly distributed in 6 spheres of four satellites each. The satellites are disposed approximately 10,900 nautical miles above the earth and maintain orbit longitudinal spacing angles of about 60 degrees from each other. The GPS satellites orbit about the centrally disposed earth. The semimajor axis of each satellite is controlled to maintain equal spacing from the earth so that the satellites pass over a given location on earth at predictable, periodic pass-by times, e.g., regularly in 12 hour intervals. Thus, each GPS satellite concludes a complete orbit twice daily. Thus, assuming a complete constellation of GPS satellites, an average of 4.8 satellites would be in view at any given time from any given location on earth, notwithstanding signal obscuration by trees, mountains, buildings and other natural and manmade obstacles.
The former Soviet Union had been launching similar positioning satellites dubbed "GLONASS." There is a strong possibility that the GPS and GLONASS systems may be combined into one mega-constellation of positioning satellites. Accordingly, the invention herein contemplates the usage of GLONASS and other like systems.
The position of each GPS satellite in its orbit may be precisely determined. Each satellite includes an atomic clock, whereby the time at which a signal is transmitted from that satellite is precisely known. The object, whose latitude and longitude on earth is to be tracked, includes a ground GPS receiver for receiving and processing these satellite signals. A ground GPS receiver also includes a clock and a computer processing unit (CPU), which together are capable of determining the propagation time, i.e., the time required for signals to be propagated from the satellite to the ground GPS receiver, and therefore is capable of calculating the distance between each of at least three satellites and the ground GPS receiver to thereby accurately determine by well known triangulation techniques its position in terms of latitude and longitude on earth. In particular, the distance between a particular satellite and the ground GPS receiver is the product of the velocity of light, i.e., 186,000 miles per second, and the determined propagation time. To calculate object location it is also necessary to know accurately the positions of the satellites. The ground GPS receivers store therein data indicative of the continuously changing positions of all of the active satellites in the GPS system. Such data is transmitted by each satellite to the ground GPS receivers to use in these object location calculations. When signals from three satellites are received by a ground GPS receiver, a 2-dimensional position, i.e., latitude and longitude, may be determined. When signals from four satellites are received by the GPS receiver, a 3-dimensional position, i.e., latitude, longitude and altitude, may be determined.
The Department of Defense operates its GPS system to provide two distinct services. The first or Precise Positioning Service (PPS) is reserved for military use and is believed to be capable of determining object location to an accuracy of at least one meter. A second, less precise system known as the Standard Positioning Service (SPS) is available for general civilian use.
The accuracy of the propagation time determination and therefore the calculations of the distances between the ground GPS receiver and each of the overhead satellites, is dependent directly on the accuracy of the clock included in the ground GPS receiver. The accuracy of the receiver clock is maintained by synchronizing it with the operation of the satellite's atomic clock by transmitting a binary pseudo-random code from each satellite to the ground GPS receiver. As will be explained, the Precise Positioning Service and the Standard Positioning Service use different methods and pseudo-random codes for synchronizing the receiver clocks.
The accuracy of the object location calculations is thus dependent upon the accuracy of the clock of the ground GPS receiver. To calculate position location to an accuracy of one meter, the ground receiver clock and therefore the calculation of the propagation times require an accuracy of better than 100 ns. To maintain receiver clock accuracy, the satellites transmit timing marks at approximately one microsecond intervals. The ground receivers' clocks differ from the satellite clocks by an error or clock bias C.sub.B. Dependent upon the error or bias C.sub.B of the ground receiver clock, the object location calculations performed by the ground receivers are all in error by a fixed amount, which is called a pseudo-range "n".
Relative uncertainties in the calculations of object location by the ground GPS receiver occur because of several factors such as ionospheric delays, ambient temperature fluctuations and Doppler shift. Such uncertainties are expressed collectively as the dilution of precision. The Department of Defense increases the dilution of precision when it implements a policy of unscheduled Selective Availability in its Standard Positioning Service, which causes the calculated object location to appear off by the pseudo-range n, where n is whatever the Department of Defense selects, but generally, not in excess of such a value where n would cause an inaccuracy over 100 meters. The Department of Defense uses Selective Availability to prevent potential aggressors against the United States to employ the GPS system in a potential attack. However, Selective Availability, especially when combined with those elements contributing to normal dilution of precision, could prove to be detrimental to civilian uses of the Standard Positioning Service, inducing varying errors into the calculations of object location by ground GPS receivers.
Each satellite transmits at a rate of 50 bps a tri-group of data in a direct sequence spread spectrum (DSSS) form, containing therein information concerning the almanac, ephemeris, and clock correction. The almanac, which is generally reliable for a period of at least 30 days, contains general information regarding the position of the entire GPS constellation. The ephemeris is satellite-specific progression and path information, which is generally reliable for up to 120 minutes (the duration of time during which the geographical footprint generated by reliable signals made on earth from a satellite vehicle is of sufficient strength to reliably participate in a positioning fix). The clock correction parameters are necessary because even atomic clocks are not perfect and such timing offsets, while greatly compensated for with ground relayed referencing to the National Observatory time standardization in the District of Columbia, may be further corrected with user-corrected referencing. The satellites transmit their signals in both the Precise and Standard Positioning Services on a common carrier frequency within the L-band's upper limit at 1575.42 MHz (L1), carrying with this frequency two distinct, binary pseudo-random codes emitted at two chip rates corresponding respectively to the Precise Positioning Service and the Standard Positioning Service. The chip rate for the Precise Positioning Service is 10.23 MHz, which is associated with a Precise or P-code. In the case of the Standard Positioning Service, a pseudo-random noise signal (PRN), which has a chip rate of 1.023 MHz and is unique to each satellite, is used to spread the spectrum of the transmitted information about the center frequency. The pseudo-random noise signal is known as a coarse/acquisition (C/A) code since it provides the timing marks required for fast acquisition of GPS signals and coarse navigation. Each satellite has a different spread spectrum access code for both a clear acquisition (C/A) and a precision (P) code. The C/A code is a pseudo-random string of ones and zeros applied to a device which controls the carrier phase in 180 degree increments. This technique is known as bi-phase direct sequence spread spectrum at the 1.023 MHz chip rate. The P code is much longer in length and is applied at the 10.23 MHz chip rate. Details of the GPS are given in NAVIGATION: Journal of the institution of Navigation, Vol. 25, No. 2, December 1978. The satellites repeatedly transmit at 1-millisecond intervals their pseudo-random codes to the ground GPS receivers. The signals received at a ground receiver have a bandwidth of approximately 2 MHz and a signal-to-noise ratio (S/N) of approximately -20 db.
Since the satellites are each moving at a speed in excess of 3 km/s, the GPS signals are received with a Doppler frequency offset from the GPS center frequency. As a result, a stationary ground GPS receiver has to be capable of receiving signals with frequencies of up to + or -4 KHz from the GPS center frequency, and a mobile receiver (as is usually the case) has to be able to receive signals over an even greater frequency range. To recover the data and measure the propagation time of the satellite signals, the ground GPS receiver must compensate for the Doppler frequency offset and also synchronize its clock with the atomic clock of a satellite by generating the C/A code corresponding to each satellite. In particular, the ground GPS receiver must generate a replica of the pseudo code transmitted from the satellite for control of an internal phase switch and synchronize the code in time with the code received at its antenna in order to recover the carrier frequency. The code time with respect to the receiver's clock is measured for four satellites and used for determining the position of the GPS receiver on the earth. See, for example, U.S. Pat. Nos. 4,457,006 and 4,114,155. Initially, at least, this synchronizing can be very time consuming since to despread the DSSS signals, the incoming and locally generated PRN code delay, the ground GPS receiver must compare the locally generated code and the incoming code at a number of different positions until the point of synchronism or correlation is found. With a code length of 1023 chips this comparison can be a lengthy procedure. However, once the frequency offset and the PRN code delay for each satellite are known, tracking them is relatively easy.
U.S. Pat. No. 4,983,980 contemplates the mounting of a GPS receiver on a vehicle, for determining the location of that vehicle as it moves from place to place. This patent contemplates that such a vehicle may pass through a tunnel, whereby the GPS receiver may lose the transmission of the GPS signals from the satellites. Even after the vehicle emerges from the tunnel, it takes time for the vehicle's GPS receiver to reestablish reception of the satellite signal. In particular, GPS satellites continuously rotate about the earth, whereby the center frequency of the satellite signal is shifted due to the Doppler effect when received by the ground GPS receiver disposed at a relatively stationary position on the earth. The ground GPS receiver initiates receiving of the spread-spectrum signal from the satellite by locking a phase-locked loop (PLL) circuit of the GPS receiver to the center frequency of the GPS signal which may be shifted by the Doppler effect. Upon locking of the PLL circuit, the spread-spectrum signal is despread and demodulated to receive the GPS signal. Thus even after the vehicle emerges from the tunnel and its GPS receiver again has a line of sight contact with an overhead signal, the GPS receiver of the vehicle requires some delay before the satellite signal is received and demodulated and may again start calculating the vehicle position. This patent discloses a ground GPS receiver, which comprise a clock and a random access memory for storing the latitude and longitude of a last-known location, e.g., the latitude and longitude of Tokyo when the vehicle is driven in Japan, and for using the almanac information of each GPS satellite to determine the position of the satellites, when the vehicle reemerges into direct line of sight with the satellites. In particular, the GPS receiver identifies the strongest satellites at the highest mask angle (reference to the horizonal plane) at the time when the vehicle reappears from the tunnel and has a direct line of sight with the satellites.
In those applications where a GPS receiver is mounted on a vehicle, the receiver may be used for security application. For example, the GPS receiver may continue to calculate the vehicle location and to transmit that location to a distant point, where location data may be used by the police to track the vehicle. For example, if the vehicle is stolen, the vehicle owner or, preferably, the police could use the vehicle location to retrieve the vehicle, apprehend the thief and to discourage the theft of the vehicle, in the first instance. In potential security applications as well as in everyday tracking of the vehicle, the vehicle may be taken to places, wherein its GPS receiver may no longer receive satellite signals. For example, the vehicle may be taken into an underground garage. Vehicles may be kept in such places for hours or even days and then emerge so that its GPS receiver may again reacquire transmission of the satellite signals and to again calculate the vehicle's location.
U.S. Pat. Nos. 5,043,736 and 5,119,102 disclose the combination of a GPS receiver and a transmitter for transmitting GPS system data from the receiver to a remote base station. The '736 patent suggests that the transmitter be implemented by cellular system technology.
U.S. Pat. No. 4,751,512 suggests improving the accuracy provided by a GPS system operated in the Standard Positioning Service by operating such a system in a so called "differential mode". Generally, operation in differential mode involves combining navigational information received at two different receivers, where the location of one of the receivers is known. By combining the data, the location the other receiver can be determined with greater accuracy than would be possible through using the data received by that other receiver alone. In particular, a GPS receiver may be disposed at a known location to determine the difference between its known location and its location predicted based upon receiving the satellite signals and calculating therefrom the approximate location. This difference reflects errors in the information received including those deliberately induced by the Department of Defense in its Standard Positioning Service. This differential data must be communicated from the reference receiver to a user, who is typically displaced from the reference station. The '512 patent particularly suggests that the associated transmitting unit transmit the differential data via a commercial geosynchronous earth satellite relay to a user located no more than 500 miles from the reference receiver.